(1) Field of the Invention
The present invention relates to an optical transmitter used in an optical communication system, and for example, to an optical transmitter provided with a directly modulated laser.
(2) Description of Related Art
An optical transmitter module provided with a directly modulated laser (DML) is simple in comparison with one using an external modulator and has advantages that the cost is low, the space is saved, and the consumed power is small.
However, the DML has large wavelength chirping in comparison with one using an external modulator and has a disadvantage that the transmission distance is reduced as a result.
Here, the wavelength chirping means wavelength fluctuations that occur accompanying intensity modulation of light. In order to perform long distance transmission, it is necessary to suppress the wavelength chirping low.
Recently, there is discussion about an attempt to suppress the wavelength chirping low in order to perform longer distance transmission using the DML by optimizing drive methods and performing signal spectrum processing by use of a wavelength filter (for example, refer to D. Mahgerefteh et al. “Error-free 250 km transmission in standard fibre using compact 10 Gbit/s chirp-managed directly modulated lasers (CML) at 1550 nm” ELECTRONICS LETTERS 28 Apr. 2005 Vol. 41 No. 9).
In this specification, such a DML is particularly referred to as a low chirp directly modulated laser or low-chirp DML.
Here, FIG. 18 shows a configuration of a low chirp directly modulated laser in the prior art.
As shown in FIG. 18, the low chirp directly modulated laser in the prior art uses a single oscillation mode DFB laser 100 as a directly modulated semiconductor laser and the laser light output from the DFB laser 100 (DFB laser output light; modulated light) is caused to be inputted to a wavelength filter 103 via a coupling lens 101 and an isolator 102.
In this case, part of filter input light inputted to the wavelength filter 103 passes through the wavelength filter 103 and is output as filter transmitted light (module output light) as a result. On the other hand, part of the filter input light is cut off by the wavelength filter 103 and returns to the isolator 102 side as filter cutoff light.
Additionally, the isolator 102 is arranged in order to prevent the cutoff light cut off by the wavelength filter 103 from entering the DFB laser 100 as returned light.
The drive conditions of the DFB laser 100 in the low chirp directly modulated laser, the filter conditions of the wavelength filter 103, etc., will be explained in detail below with the principles of the low chirp directly modulated laser. Here, there is exemplified the case where the low chirp directly modulated laser is driven under the conditions that the operation wavelength is 1.55 μm and the modulation rate is 10 Gb/s.
By comparing with a normal DML, the DFB laser 100 in the low chirp directly modulated laser is driven under the conditions that the average light output is increased by increasing the value of the direct current bias and the extinction ratio is reduced by reducing the drive current amplitude value. Normally, the extinction ratio is set to about 2 dB in the low chirp directly modulated laser.
Here, FIG. 19 schematically shows the light intensity waveform of laser light output from the DFB laser 100 when the DFB laser 100 is driven under such drive conditions.
On the basis of the properties of the DFB laser 100, the wavelength of output light from the DFB laser 100 generates time fluctuation as shown in FIG. 20 in accordance with the light intensity waveform shown in FIG. 19 in the case of such high average light output and low extinction ratio drive (this is referred to as a wavelength chirp waveform). In other words, the laser light (output light) output from the DFB laser 100 changes in wavelength approximately in proportion to the light intensity waveform (the proportion coefficient is a negative value).
For example, when the light intensity of laser light output from the DFB laser 100 is high, that is, in the case of the ON level, the wavelength of the laser output light becomes shorter (the frequency becomes higher) and conversely, when the light intensity of laser light output from the DFB laser 100 is low, that is, in the case of the OFF level, the wavelength of the laser output light becomes longer (the frequency becomes lower). Normally, in a low chirp directly modulated laser, the drive current amplitude value and the direct current bias current value are set such that the difference in wavelength of the laser output light between the ON level and the OFF level becomes about 5 GHz in terms of the frequency.
Additionally, in the case of the normal DML (not the low chirp directly modulated laser type), not only the change in wavelength in proportion to the light intensity waveform but also a large wavelength fluctuation component in proportion to the time derivative thereof are produced in the wavelength chirp (fluctuation) waveform in FIG. 20.
In contrast to this, in the case of the low chirp directly modulated laser, the wavelength fluctuation component in proportion to the time derivative of such a light intensity waveform is suppressed low by being driven under the conditions that the average light output is increased and the extinction ratio is suppressed low, which results in making a signal more suitable for optical fiber transmission.
By the way, the spectrum of the output light from the DFB laser in the low chirp directly modulated laser is such one as shown by the solid line A in FIG. 21.
Additionally, in the spectrum shown by the solid line A in FIG. 21, the short wave component corresponds to the ON level component of the output light from the DFB laser and the long wave component corresponds to the OFF level component of the output light from the DFB laser, respectively.
In the low chirp directly modulated laser, signal spectrum processing is performed for the output light spectrum of such a DFB laser by use of a wavelength filter having a wavelength filter function as shown by the solid line B in FIG. 21. In other words, filtering processing is performed such that the short wave component corresponding to the ON level of the light intensity waveform is transmitted and the long wave component corresponding to the OFF level is cut off.
With such filtering processing, the signal spectrum of the wavelength filter transmitted light after passing through the filter and the light intensity waveform of the wavelength filter transmitted light become those shown in FIGS. 22 and 23, respectively.
In other words, as shown in FIG. 22, the signal light of the OFF level component is cut off relatively more than the signal light of the ON level component, therefore, as shown in FIG. 23, the light intensity waveform of the wavelength filter transmitted light after passing through the filter has as large a extinction ratio as about 10 dB in comparison with the light intensity waveform of the output light from the DFB laser shown in FIG. 19. Further, as shown in FIG. 22, the width of the signal spectrum becomes small because the OFF level component has been cut off.
As described above, in the low chirp directly modulated laser, the spectrum width of the signal light is reduced using the wavelength filter so that it becomes more unlikely to receive the influence of the group velocity dispersion of optical fiber transmission, and long-distance transmission is made possible, as a result. Further, since the extinction ratio of the light intensity waveform to be output finally via the wavelength filter is large, it is possible to use the low chirp directly modulated laser for normal and simple intensity modulation on off keying (IM-OOK).